The bearing apparatus for vehicle wheel is adapted to freely rotatably support a wheel hub to mount the wheel, via a rolling bearing. An inner ring rotation type is used for a driving wheel and both inner ring rotation and outer ring rotation types for a driven wheel. A double row angular ball bearing is widely used in such a bearing apparatus. Reasons for this is that it has a desirable bearing rigidity, high durability against misalignment and small rotation torque required for fuel consumption. The double row angular contact ball bearing has a plurality of balls interposed between a stationary ring and a rotational ring. The balls are contacted with the stationary and rotational rings with a predetermined contact angle.
The vehicle wheel bearing apparatus is broadly classified into a first, second and third generation type. Structure of the first generation includes a wheel bearing of double row angular contact ball bearing fit between a knuckle that forms part of a suspension and a wheel hub. Structure of the second generation includes a body mounting flange or a wheel mounting flange directly formed onto the outer circumferential surface of an outer member (outer ring). Structure of the third generation includes one of the inner raceway surfaces directly formed on the outer circumferential surface of the wheel hub.
Recently, there has been a strong desire for the bearing apparatus for a vehicle wheel not only to improve durability and cost reduction but to improve NVH (i.e. Noise, Vibration and Harshness). FIG. 7 illustrates a prior art wheel bearing 50 used for a vehicle wheel bearing apparatus. The bearing 50 has a double row angular contact ball bearing comprising an outer ring 51 formed on its inner circumferential surface with double row outer raceway surfaces 51a, 51a. A pair of inner rings 52, 52 are each formed on their outer circumferential surface with an inner raceway surface 52a oppositely facing each of the outer raceway surfaces 51a, 51a. A plurality of balls 53, 53 is contained between the inner and outer raceway surfaces. Cages 54 rotatably hold the balls 53. Seals 55, 56 are arranged in an annular space between the outer ring 51 and inner rings 52, 52 to prevent leakage of lubricating grease sealed within the bearing and ingress of dust or rain water into the bearing from the outside.
Such a bearing 50 is called a first generation bearing. The bearing 50 has counter portions (projections) 57, shown in an enlarged view of FIG. 8, formed near the bottom of the inner raceway surfaces 52a of the inner rings 52. Additionally, the bearing has an outer diameter larger than a diameter (d1) of the bottom of the inner raceway surface 52a. Accordingly, balls 53 interfere with the counter portions 57 when the inner rings 52 are moved axially. Thus, the balls 53 coming out of the inner rings 52 is prevented by the counter portions 57. That is, the outer diameter (d2) of the counter portion 57 of the inner ring 52 is larger than the inscribed circle diameter (d0) of the balls under a supposed condition where the balls 53 are perfectly held within the outer raceway surface 51a as if they would be contacted with the bottom of the outer raceway surface 51a. Accordingly a so-called “run-over height” δ (one side) is provided.
In addition, all of the outer circumferential surface of a shoulder 52b of the inner ring 52, the inner raceway surface 52a, the counter portion 57, and a small end face 52c are simultaneously ground by a formed grinding wheel. Furthermore, attempts have been made to minimize a setting range of the initial gap and to reduce the dispersion amount of bearing preload by minimizing the respective dimensional dispersion as well as by limiting the run-over height δ and the central position deviation (L) (i.e., a distance between the bottom of the inner raceway surface 52a and the small end face 52c) to a predetermined value range (see e.g. Japanese laid-open Patent publication No. 193745/2001).